The present invention relates generally to nuclear reactor fuel assemblies and more particularly to a reusable system for removably attaching a nuclear reactor fuel assembly duct tube to a nuclear reactor fuel assembly inlet nozzle.
To produce power from the nuclear reactor, it is necessary to assemble a concentration of fissionable uranium, thorium, and/or plutonium in a quantity and in a physical configuration capable of sustaining a continuous sequence of fission reactions. This concentration is frequently referred to as the reactor core. The heat that the fission reactions generate is transferred to a fluid such as, for example, liquid sodium. The sodium, in turn, transfers its heat to a secondary coolant, or ultimate coolant, which can be used to drive turbines which power electrical generation equipment.
Because the radiation, pressure, temperature, flow velocity, and other environmental conditions within the reactor core are quite hostile, the reactor core must be of sturdy construction. These conditions produce a number of phenomena with which it is very difficult to cope. Thus, for example, thermal, pressure, and irradiation effects tend to produce considerable creep in materials which can expand dimensions, cause bow and other effects which shorten fuel assembly life. Also, due to varying material requirements throughout a reactor core, it is often difficult to achieve ideal fuel behavior due to fabrication constraints. These effects have been known to produce a type of deterioration in that dimensional constraints of a fuel assembly in a reactor core can no longer be met over a period of time.
These conflicting requirements have been reconciled to a great extent by loading pellets of uranium or plutonium dioxide into long slender tubes called fuel rods. With the tubes loaded with pellets and the ends of each tube sealed, these fuel rods are arranged longitudinally parallel with each other and are arranged in generally hexagonal arrays of about 200 fuel rods. Each array is called a fuel assembly. These fuel assemblies all are mounted side-by-side in a larger, generally right circular cylindrical configuration that characterizes the reactor core.
Each fuel assembly often includes an upper handling socket and a lower nozzle which are both attached to a duct tube containing the fuel rods. The fuel rods receive radial support from the duct tube and longitudinal support from the lower nozzle. Coolant usually flows in through the lower or inlet nozzle of the fuel assembly, up through the spaces between fuel rods and out through the upper handling socket of the fuel assembly.
Often, duct material incompatibility with the environment can degrade the operating life expectancy of the fuel assembly. This gives a need, with a significant cost incentive, for being able to remove duct tubes from their inlet nozzles for various reasons such as inspection, testing, maintenance or replacement, without the destruction of the duct tube and/or inlet nozzle.
The prior art attaches the duct tube to the inlet nozzle by welding, and uses destructive techniques to separate them.